l Init进程
Android系统在启动时首先会启动Linux系统，引导加载Linux Kernel并启动init进程。Init进程是一个由内核启动的用户级进程，是Android系统的第一个进程。该进程的相关代码在platform\system\core\init\init.c。在main函数中，有如下代码：
open_devnull_stdio(); log_init(); INFO("reading config file\n"); init_parse_config_file("/init.rc"); /* pull the kernel commandline and ramdisk properties file in */ import_kernel_cmdline(0); get_hardware_name(hardware, &revision); snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware); init_parse_config_file(tmp);
这里会加载解析init.rc和init.hardware.rc两个初始化脚本。*.rc文件定义了在init进程中需要启动哪些进程服务和执行哪些动作。其详细说明参见platform\system\core\init\reademe.txt。init.rc见如下定义：
service servicemanager /system/bin/servicemanager user system critical onrestart restart zygote onrestart restart mediaservice vold /system/bin/vold socket vold stream 0660 root mount ioprio be 2service netd /system/bin/netd socket netd stream 0660 root system socket dnsproxyd stream 0660 root inetservice debuggerd /system/bin/debuggerdservice ril-daemon /system/bin/rild socket rild stream 660 root radio socket rild-debug stream 660 radio system user root group radio cache inet misc audio sdcard_rwservice zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server socket zygote stream 666 onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart media onrestart restart netdservice drm /system/bin/drmserver user drm group system root inet
具体解析过程见platform\system\core\init\Init_parser.c。解析所得服务添加到service_list中，动作添加到action_list中。
接下来在main函数中执行动作和启动进程服务：
execute_one_command(); restart_processes()
通常init过程需要创建一些系统文件夹并启动USB守护进程、Android Debug Bridge守护进程、Debug守护进程、ServiceManager进程、Zygote进程等。
l ServiceManager进程
ServiceManager进程是所有服务的管理器。由init.rc对ServiceManager的描述service servicemanager /system/bin/servicemanager可知servicemanager进程从platform\frameworks\base\cmd\servicemanager\Service_manager.cpp启动。在main函数中有如下代码：
int main(int argc, char **argv){ struct binder_state *bs; void *svcmgr = BINDER_SERVICE_MANAGER; bs = binder_open(128*1024); if (binder_become_context_manager(bs)) { LOGE("cannot become context manager (%s)\n", strerror(errno)); return -1; } svcmgr_handle = svcmgr; binder_loop(bs, svcmgr_handler); return 0;}
首先调用binder_open()打开Binder设备(/dev/binder)，调用binder_become_context_manager()把当前进程设置为ServiceManager。ServiceManager本身就是一个服务。
int binder_become_context_manager(struct binder_state *bs){ return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);}
最后binder_loop()进入循环状态，并设置svcmgr_handler回调函数等待添加、查询、获取服务等请求。
l Zygote进程
Zygote进程用于产生其他进程。由init.rc对zygote的描述service zygot /system/bin/app_process可知zygote进程从platfrom\frameworks\base\cmds\app_process\App_main.cpp启动。在main函数中有如下代码：
if (0 == strcmp("--zygote", arg)) { bool startSystemServer = (i < argc) ? strcmp(argv[i], "--start-system-server") == 0 : false; setArgv0(argv0, "zygote"); set_process_name("zygote"); runtime.start("com.android.internal.os.ZygoteInit", startSystemServer); } else { set_process_name(argv0); runtime.mClassName = arg; // Remainder of args get passed to startup class main() runtime.mArgC = argc-i; runtime.mArgV = argv+i; LOGV("App process is starting with pid=%d, class=%s.\n", getpid(), runtime.getClassName()); runtime.start(); }
首先创建AppRuntime，即AndroidRuntime，建立了一个Dalvik虚拟机。通过这个runtime传递com.android.internal.os.ZygoteInit参数，从而由Dalvik虚拟机运行ZygoteInit.java的main()，开始创建Zygote进程。在其main()中，如下所示：
registerZygoteSocket(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis()); preloadClasses(); //cacheRegisterMaps(); preloadResources(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis()); // Finish profiling the zygote initialization. SamplingProfilerIntegration.writeZygoteSnapshot(); // Do an initial gc to clean up after startup gc(); // If requested, start system server directly from Zygote if (argv.length != 2) { throw new RuntimeException(argv[0] + USAGE_STRING); } if (argv[1].equals("true")) { startSystemServer(); } else if (!argv[1].equals("false")) { throw new RuntimeException(argv[0] + USAGE_STRING); }
首先通过registerZygoteSocket()登记端口，接着preloadClasses()装载相关类。这里大概要装载1000多个类，具体装载类见platform\frameworks\base\preloaded-classes。这个文件有WritePreloadedClassFile类自动生成。分析该类的main函数，有如下一段筛选类的代码：
// Preload classes that were loaded by at least 2 processes. Hopefully, // the memory associated with these classes will be shared. for (LoadedClass loadedClass : root.loadedClasses.values()) { Set<String> names = loadedClass.processNames(); if (!Policy.isPreloadable(loadedClass)) { continue; } if (names.size() >= MIN_PROCESSES || (loadedClass.medianTimeMicros() > MIN_LOAD_TIME_MICROS && names.size() > 1)) { toPreload.add(loadedClass); } } int initialSize = toPreload.size(); System.out.println(initialSize + " classses were loaded by more than one app."); // Preload eligable classes from applications (not long-running // services). for (Proc proc : root.processes.values()) { if (proc.fromZygote() && !Policy.isService(proc.name)) { for (Operation operation : proc.operations) { LoadedClass loadedClass = operation.loadedClass; if (shouldPreload(loadedClass)) { toPreload.add(loadedClass); } } } }
其中MIN_LOAD_TIME_MICROS等于1250，当类的装载时间大于1.25ms，则需要预装载。
Policy.isPreloadable()定于如下：
/**Reports if the given class should be preloaded. */ public static boolean isPreloadable(LoadedClass clazz) { return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name); }
其中EXCLUDED_CLASSES如下定义：
/** * Classes which we shouldn't load from the Zygote. */ private static final Set<String> EXCLUDED_CLASSES = new HashSet<String>(Arrays.asList( // Binders "android.app.AlarmManager", "android.app.SearchManager", "android.os.FileObserver", "com.android.server.PackageManagerService$AppDirObserver", // Threads "android.os.AsyncTask", "android.pim.ContactsAsyncHelper", "java.lang.ProcessManager" ));
这几个Binders和Thread是不会被预加载的。
另外还有一些application需要装载，要求满足条件proc.fromZygote()且不是属于常驻内存的服务。SERVICES定义如下：
/** * Long running services. These are restricted in their contribution to the * preloader because their launch time is less critical. */ // TODO: Generate this automatically from package manager. private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList( "system_server", "com.google.process.content", "android.process.media", "com.android.bluetooth", "com.android.calendar", "com.android.inputmethod.latin", "com.android.phone", "com.google.android.apps.maps.FriendService", // pre froyo "com.google.android.apps.maps:FriendService", // froyo "com.google.android.apps.maps.LocationFriendService", "com.google.android.deskclock", "com.google.process.gapps", "android.tts" ));
preloaded-classes是在下载源码的时候生成，WritePreloadedClassFile类并没有被用到，但可以通过这个类了解Android系统对预加载类的默认要求，参考修改preloaded-classes文件，减少开机初始化时要预加载的类，提高开机速度。
最后来通过startSystemServer()启动SystemServer进程。见如下代码：
/* Hardcoded command line to start the system server */ String args[] = { "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003", "--capabilities=130104352,130104352", "--runtime-init", "--nice-name=system_server", "com.android.server.SystemServer", }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteConnection.Arguments(args); /* * Enable debugging of the system process if *either* the command line flags * indicate it should be debuggable or the ro.debuggable system property * is set to "1" */ int debugFlags = parsedArgs.debugFlags; if ("1".equals(SystemProperties.get("ro.debuggable"))) debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER; /* Request to fork the system server process */ pid = Zygote.forkSystemServer( parsedArgs.uid, parsedArgs.gid, parsedArgs.gids, debugFlags, null, parsedArgs.permittedCapabilities, parsedArgs.effectiveCapabilities)
Zygote包装了Linux的fork。forkSystemServer()调用forkAndSpecialize()，最终穿过虚拟机调用platform\dalvik\vm\native\dalvik_system_Zygote.c中Dalvik_dalvik_system_Zygote_forkAndSpecialize()。由dalvik完成fork新的进程。
main()最后会调用runSelectLoopMode()，进入while循环，由peers创建新的进程。
l SystemService进程
SystemService用于创建init.rc定义的服务之外的所有服务。在main()的最后有如下代码：
// The system server has to run all of the time, so it needs to be // as efficient as possible with its memory usage. VMRuntime.getRuntime().setTargetHeapUtilization(0.8f); System.loadLibrary("android_servers"); init1(args);
Init1()是在native空间实现的，用于启动native空间的服务，其实现在com_android_server_SystemServer.cpp中的android_server_SystemServer_init1()：
static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz){ system_init();}
而system_init()服务初始化创建native层的各个服务：
// Start the sensor service SensorService::instantiate(); // On the simulator, audioflinger et al don't get started the // same way as on the device, and we need to start them here if (!proc->supportsProcesses()) { // Start the AudioFlinger AudioFlinger::instantiate(); // Start the media playback service MediaPlayerService::instantiate(); // Start the camera service CameraService::instantiate(); // Start the audio policy service AudioPolicyService::instantiate(); }
最后通过如下代码:
LOGI("System server: starting Android services.\n"); runtime->callStatic("com/android/server/SystemServer", "init2");
回到SystemServer.java，调用init2()：
public static final void init2() { Slog.i(TAG, "Entered the Android system server!"); Thread thr = new ServerThread(); thr.setName("android.server.ServerThread"); thr.start(); }
Init2启动一个线程，专门用来启动java空间的所有服务。如下代码所示启动部分服务：
Slog.i(TAG, "Content Manager"); ContentService.main(context, factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL); Slog.i(TAG, "System Content Providers"); ActivityManagerService.installSystemProviders(); Slog.i(TAG, "Battery Service"); battery = new BatteryService(context); ServiceManager.addService("battery", battery); Slog.i(TAG, "Lights Service"); lights = new LightsService(context); Slog.i(TAG, "Vibrator Service"); ServiceManager.addService("vibrator", new VibratorService(context)); // only initialize the power service after we have started the // lights service, content providers and the battery service. power.init(context, lights, ActivityManagerService.getDefault(), battery); Slog.i(TAG, "Alarm Manager"); AlarmManagerService alarm = new AlarmManagerService(context); ServiceManager.addService(Context.ALARM_SERVICE, alarm);
并且把这些服务添加到ServiceManager中，以便管理和进程间通讯。
在该线程后半部分，ActivityManagerService会等待AppWidget、WallPaper、IMM等systemReady后调用自身的systemReady()。
((ActivityManagerService)ServiceManager.getService("activity")) .setWindowManager(wm); // Skip Bluetooth if we have an emulator kernel // TODO: Use a more reliable check to see if this product should // support Bluetooth - see bug 988521 if (SystemProperties.get("ro.kernel.qemu").equals("1")) { Slog.i(TAG, "Registering null Bluetooth Service (emulator)"); ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null); } else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) { Slog.i(TAG, "Registering null Bluetooth Service (factory test)"); ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null); } else { Slog.i(TAG, "Bluetooth Service"); bluetooth = new BluetoothService(context); ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth); bluetooth.initAfterRegistration(); bluetoothA2dp = new BluetoothA2dpService(context, bluetooth); ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE, bluetoothA2dp); int bluetoothOn = Settings.Secure.getInt(mContentResolver, Settings.Secure.BLUETOOTH_ON, 0); if (bluetoothOn > 0) { bluetooth.enable(); } }
而在ActivityManagerService的systemReady()最后会执行如下代码：
mMainStack.resumeTopActivityLocked(null);
由于Activity管理栈为空，因此启动Launcher。
// Find the first activity that is not finishing. ActivityRecord next = topRunningActivityLocked(null); // Remember how we'll process this pause/resume situation, and ensure // that the state is reset however we wind up proceeding. final boolean userLeaving = mUserLeaving; mUserLeaving = false; if (next == null) { // There are no more activities! Let's just start up the // Launcher... if (mMainStack) { return mService.startHomeActivityLocked(); } }
在startHomeActivityLocked()中创建一个带Category为CATEGORY_HOME的Intent，由此去启动相应Activity，即Launcher。
Intent intent = new Intent( mTopAction, mTopData != null ? Uri.parse(mTopData) : null); intent.setComponent(mTopComponent); if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) { intent.addCategory(Intent.CATEGORY_HOME); }
这样，Android系统便启动起来进入到待机界面。
来源：http://www.cnblogs.com/zhixingwujiang/archive/2011/07/25/2116650.html